1. Field of the Invention
The present invention relates to an optical sheet and backlight assembly, and more particularly, to optical sheets for optically enhancing brightness characteristics and view angle characteristics.
2. Description of the Related Art
Unlike other display devices, liquid crystal (LC) molecules interposed between a thin film transistor (TFT) substrate and a color filter substrate of an LCD device are not light-emitting materials emitting light in themselves, but light-receiving materials controlling an amount of light coming from the outside to display an image. Accordingly, the LCD device indispensably requires a separate device for illuminating light onto an LC display panel, i.e., a backlight assembly.
A backlight assembly may typically include: a mold frame in which a receiving space is formed; a reflecting sheet installed on a lower surface of the receiving space to reflect light to an LC display panel; a light guide plate installed on an upper surface of the reflecting sheet to guide light; a lamp unit installed between the light guide plate and lateral walls of the receiving space to emit light; optical sheets stacked on an upper surface of the light guide plate to diffuse and condense light; and a top chassis installed on an upper portion of the mold frame to cover a region ranging from a predetermined location at edges of the LC display panel to a lateral side of the mold frame.
The optical sheets typically may include: a diffusion sheet for diffusing light; a prism sheet stacked on an upper surface of the diffusing sheet to condense diffused light and deliver the condensed light to the LC display panel; and a protection sheet for protecting the diffusion sheet and the prism sheet.
FIG. 1 is a sectional view illustrating a construction of a related art LCD device.
Referring to FIG. 1, the related art LCD device 60 includes a backlight assembly 50 for generating light, and a display unit 40 provided on an upper surface of the backlight assembly 50 to receive light from the backlight assembly 50 and display an image.
In detail, the display unit 40 includes the LC display panel 10, and an upper polarizer 30 and a lower polarizer 20 located on an upper surface and a lower surface of the LC display panel 10, respectively. Also, the LC display panel 10 includes a TFT substrate 11 including electrodes, a color filter substrate 12, and an LC layer (not shown) interposed between the TFT substrate 11 and the color filter substrate 12.
The backlight assembly 50 includes a lamp unit 51 for generating light, and a light guide unit for guiding light generated by the lamp unit 51 to an LC display panel 10.
The lamp unit 51 includes a lamp 51a for generating light, and a lamp reflector 51b surrounding the lamp 51a. Light generated by the lamp 51a is incident to a light guide plate 52 of the light guide unit. The lamp reflector 51b reflects light generated by the lamp 51a to the light guide plate 52, thereby increasing an amount of light incident to the light guide plate 52.
The light guide unit includes a reflector 54, the light guide plate 52, and optical sheets 53. The light guide plate 52 is provided on one side of the lamp unit 51 to guide light from the lamp unit 51.
The reflector 54 is provided on a lower surface of the light guide plate 52 to reflect light leaking from the light guide plate 52 back to the light guide plate 52.
In addition, a plurality of optical sheets 53 for enhancing efficiency of light guided by the light guide plate 52 are provided on an upper surface of the light guide plate 52. In detail, the optical sheets include a diffusion sheet 53a, a prism sheet 53b, and a protection sheet 53c sequentially stacked on the upper surface of the light guide plate 52.
The diffusion sheet 53a scatters light incident from the light guide plate 52 to make a brightness distribution of light uniform.
The prism sheet 53b has an upper surface in which a triangular prism is repeatedly formed to condense light diffused by the diffusion sheet 53a in a direction perpendicular to a plane of the LC display panel 10. Accordingly, most of light passing through the prism sheet 53b propagates in a direction perpendicular to the plane of the LC display panel 10 to have a uniform brightness distribution.
The protection sheet 53c provided on an upper surface of the prism sheet 53b protects a surface of the prism sheet 53b. 
FIGS. 2 and 3 are a sectional view and a photo of the related art prism sheet of FIG. 1, respectively. FIG. 4 is a photo of the related art diffusion sheet of FIG. 1.
Referring to FIGS. 2 to 4, the related art prism sheet 100 includes a body 110 to which light diffused by the light guide plate and the diffusion sheet is initially provided, and isosceles triangular prism-shaped protrusions 120 for allowing the diffused light to propagate in a predetermined direction. The protrusions 120 are linearly arranged in stripes on the body 110.
The isosceles triangular prism-shaped protrusions 120 have a pitch of 10-100 μm, and have a characteristic in that brightness increases and a view angle becomes narrow as an angle α of a vertical angle (generally, an acute angle) of the triangular prism decreases.
The diffusion sheet as shown in FIG. 4 scatters light incident from the light guide plate 52 to make the brightness distribution of light uniform and provide light having the uniform brightness distribution to the prism sheet 100.
Referring to FIGS. 2 and 3, in the case where the triangular prism-shaped protrusion 120 is formed to face a front side, that is, in the case where the protrusion 120 faces the LC display panel, diffused light introduced via the body 110 is refracted and condensed to the front side, but light incident to an inclined surface of the protrusion cannot contribute to brightness enhancement of the front side due to total internal reflection.
To address this problem, the triangular shape of the prism sheet 100 is formed in an isosceles triangular shape, or the prism sheet 100 is reversely arranged such that the protrusion of the prism sheet 100 faces the light guide plate as an alternative. However, even in these proposed related arts, it can be very difficult to obtain desirable results in both brightness and view angle.